1. Field of the Invention
The present invention relates to hot melt adhesive compositions. More particularly, the invention relates to hot melt adhesives which exhibit improved adhesion to difficult substrates, particularly at very low or very high temperatures.
2. Description of the Related Art
Hot-melt adhesives are thermoplastic materials which are typically solid at room temperature, and are denoted by the abbreviation HMA (hot-melt adhesives). Hot melt adhesives are widely used in industry for various applications such as product assembly, packaging, hygiene and elastic attachment, lamination, case and carton sealing, bookbinding and applications in the construction bonding, furniture, and textile industries, profile wrapping, and the like.
Various HMA's have different weaknesses relating to adhesion to certain substrates and adhesion at different temperatures. For instance, hot melt adhesives based on metallocene ethylene octene copolymers, or “mEO” adhesives, which are advantageous for their low odor, high clarity, and ease of use, often exhibit poor adhesion to difficult substrates, particularly at low temperatures such as refrigerator or freezer temperatures. This can be a problem in applications relating to corrugated and/or coated carton stock for food packaging and the like. Conversely, hot melt adhesives based on amorphous poly-alpha-olefins, known as “APAO” adhesives, often suffer from poor adhesion to difficult substrates, particularly at elevated temperatures, as well as difficulty in viscosity control while maintaining adhesive and cohesive properties required for performance. This can be a problem with applications such as film laminating, woodworking, profile wrapping, PVC edge banding, and other adhesion applications with wood, steel, and wrapping films.
It is highly undesirable for materials to lose adhesion due to an adhesive that either lacks cold temperature resistance and becomes brittle, or lacks high temperature resistance and softens. However, in achieving high temperature resistance, cold temperature resistance is often sacrificed, and vice versa. In addition, an increase in adhesion may result in a decrease in cohesion between component parts of a composition. It would therefore be desirable to formulate a hot melt adhesive composition having improved cohesion properties in addition to improved adhesion performance to difficult substrates, particularly at elevated or low temperatures. Such compositions would exhibit improved adhesion to difficult substrates at elevated and/or low temperatures, as compared to conventional hot melt adhesives. Further, certain compositions would exhibit an increase in both cohesion and adhesion when considering both shear adhesion failure temperature (SAFT) and peel adhesion failure temperature (PAFT).